Crawler tractors include an engine and engine-driven transmission controllably coupled to each of the crawler tracks through a drive sprocket. The transmission generally has two hydraulically-operated clutches, one for each track. When a clutch is engaged, its track is powered and, when disengaged (or "de-clutched"), its track is free to "coast" as the tractor coasts or under the urging of the other track if the latter is still engaged with the transmission. Vehicles of this type typically have a primary pedal per track that controls both clutch and brake operation on that track.
Previous clutch and brake control systems have provided for the engagement or disengagement of each clutch by a clutch valve operated by a predetermined hydraulic pressure in the master cylinder of the corresponding brake. Consequently, depression of the left or right brake pedal would cause application of the brake and at some predetermined master cylinder pressure, the corresponding transmission valve would be shifted to the de-clutch position.
Numerous disadvantages are present in the devices of the prior art. One of the principal and most widely-recognized problems of such devices is that they are not well-equipped to handle particular operating conditions where control of or prevention of vehicle coasting is critical.
For example, when the crawler tractor is being used on a steep slope or near a precipice-like embankment, clutch disengagement prior to significant brake application could allow coasting and pose a danger to the operator and to the tractor. On a steep slope, it would be advantageous for the operator to have both driving power and braking, the latter applied with sufficient force to prevent the tractor from rolling down the hill. When working near a steep embankment or when back-filling a foundation, good control is required to "inch" the vehicle up to the embankment or trench while at the same time avoiding running the tractor over the edge. In such operating environments, operators of crawler tractors like to be able to "drive through the brakes."
A second disadvantage of the devices of the prior art is that the brake master cylinder pressure (at which de-clutching occurs) is not readily adjustable. A way to adjust braking relative to de-clutching and as a function of pedal travel would be advantageous since degrees of vehicle coasting (if any) or "driving through the brakes" could be selected in view of the type of work being performed by the tractor.
There is a long-standing need for a clutch and brake control system whereby if required, the brakes can be applied without the loss of power which occurs when the track on either side is de-clutched. There is also a long-standing need for a device whereby brake application (relative to pedal position and de-clutching) can be readily adjusted for each track.
These needs are fulfilled by the present invention which is a clutch and brake control system permitting brake application while the crawler tracks are being powered. The present invention also allows limited adjustment of the pedal position at which brake application starts to occur relative to declutching.